The photochromic compounds are substances which display the characteristic of reversibly changing in colour and/or light transmission degree, when they are exposed to some types of electromagnetic radiation and to sun light, returning back to their initial colour and transmission state when the initial light source is removed.
The known substances displaying photochromic characteristics are many and belong to different classes of either organic or inorganic compounds such as, e.g., those described in "Photochromism, by G. H. Brown (Ed.), Volume III of Weissberger "Techniques of Organic Chemistry" Series, Wiley Interscience, New York (1971) and in "Photochromism: Molecules and Systems", by H. Durr and H. Bouas-Laurent (Ed.), Volume 40 of "Studies in Organic Chemistry" Series, Elsevier (1990).
Among the organic photochromic compounds, those belonging to the classes of spiro-indolino-oxazines, of spiro-pyrans and of chromenes are particularly known.
Said compounds are capable of giving photochromic characteristics to polymerized organic materials, used as photochromic articles as, for example, disclosed in the following commonly assigned Patent Applications: IT 22529 A/87, IT 22660 A/89, IT 19389 A/90, MI 91 A 002038, and in the following commonly assigned U.S. Pat. Nos.: 5,055,576, 5,110,922; and in U.S. Pat. Nos. 3,567,605; 5,066,818; and EP 245,020.
The photochromic compounds belonging to the three above cited classes can be suitably mixed, so as to obtain, after exposure to sun light or U.V. light, the development of a colour resulting from the composition of colours of used products. Such a colour composition is particularly useful in the preparation of organic photochromic glasses for which neutral colour shades such as green, brown and grey, are required by the market.
In general, the photochromic compounds used in the field of organic glasses in order to obtain blue or red colour,are those belonging to the class of spiro-indolino-oxazines, thanks to their good characteristics of colour development and fatigue resistance.
On the contrary, yellow and orange colours are obtained by using photochromic compounds belonging to the class of chromenes or spiro-pyrans such as disclosed, e.g., in commonly assigned Italian Patent Applications MI 92 A 002379 and MI 92 A 002492; and in U.S. Pat. Nos. 5,066,818; 4,931,221 and EP 250,193.
In fact, although photochromic compounds belonging to the class of spiro-indolino-oxazines capable of developing yellow or orange colours are known as, e.g., disclosed in U.S. Pat. No. 4,816,584, it is known as well that such compounds are difficult to prepare because they form in low yield as reported, e.g., in "Photochromism", by G. H. Brown (Ed.), Volume III of Weissberger "Techniques of Organic Chemistry" Series, Wiley Interscience, New York (1971), p. 244.
However, the photochromic compounds belonging to the class of chromenes or of spiro-pyrans also display some drawbacks. In fact, some photochromic compounds belonging to such classes display a photochromic activity only when the solution or the manufactured article which contains them are cooled down to unacceptably low temperatures.
Furthermore, some photochromic compounds belonging to the class of chromenes or of spiro-pyrans display a too limited ageing resistance as compared to the other photochromic components of the above mentioned mixtures.
A further drawback is given by the poor photochromic activity of those compounds belonging to the class of chromenes or of spiro-pyrans which are capable of developing, by absorbing light of suitable wavelength, shades of yellow colors. Such a drawback strongly limits the photochromic activity of compound mixtures suitable for developing such neutral colour shades as green, brown and grey.
In fact, it is well known that the photochromic activity of compounds belonging to classes of spiro-indolino-oxazines, of chromenes and of spiro-pyrans, cannot be arbitrarily increased by increasing the concentration of photochromic compound. Such a correlation is only possible when very low concentrations of active substance are used, whilst, beyond a certain concentration, a threshold value is reached which cannot be exceeded.
On the basis of the above, it is evident that the neutral colour shades should be obtained by limiting the concentration of the photocroms which develop the blue and red colour shades, in order to limit their photochromic effect and thus make it comparable in intensity to the one displayed by the yellow component.